This invention relates to a process for producing attrition resistant zeolitic FCC catalysts from a fine particle size, zeolite-rich material generated as a co-product in the practice of a recent process and product innovation described in U.S. Pat. No. 4,493,902, "FLUID CRACKING CATALYST COMPRISING MICROSPHERES CONTAINING MORE THAT ABOUT 40 PERCENT BY WEIGHT Y-FAUJASITE AND METHODS FOR MAKING".
The present invention differs from the process of Ser. No. 532,346, which also utilizes a fine particle size, zeolitic co-product of zeolite X catalyst manufacture, in that the fine particle size co-products of in situ zeolite catalyst manufacturing in accordance with U.S. Pat. No. 4,493,902 are of much higher zeolite content than that of the co-products utilized in practice of Ser. No. 532,346. Thus, in practice of the instant invention the fine particle size zeolitic material which is a co-product contains about 50-70% zeolite Y, as opposed to ca. 20-40% in the process of Ser. No. 532,346.
In a presently most preferred embodiment, the invention relates to the recovery of co-product zeolite-rich fines obtained in commercial practice of the embodiment of the invention substantially as described in Examples 1 and 2 of U.S. Pat. No. 4,493,902. These examples describe a process in which microspheres high in zeolite Y content and of noteworthy cracking activity and hardness are produced by immersing porous microspheres comprising a mixture of calcined clays in a solution comprising alkaline sodium silicate and a clear solution of "external" amorphous zeolite seeds. The reactants (microspheres of calcined clay and alkaline sodium silicate solution) are heated in the presence of the seeding material and zeolite Y crystals form in situ in the pores of channels in the microspheres at levels usually in the range of 50% to 70%. The entire disclosure of U.S. Pat. No. 4,493,902 is incorporated herein by cross-reference thereto. The procedure described in said patent in which seeds are incorporated into the aqueous phase of the reaction mixture will be referred hereinafter to as an externally seeded in situ process. The procedure in which seeds are a component of the unreacted microspheres, e.g., Example 3 of the patent, will be referred to as an internally seeded in situ process.
In commercial practice of the externally and internally seeded variations of the in situ processes of U.S. Pat. No. 4,493,902, discrete particles having a high content of crystalline zeolite Y may form in the aqueous reaction liquid as well as within the channel of pores and on the surfaces of the microspheres. These fines are considerably smaller than grade sized microspheres, the latter typically being 20 to 150 microns in diameter. An alkaline sodium silicate effluent containing finely divided zeolite-rich particles is formed. This represents a waste of zeolite as well as a disposal expense since the pH is typically 11.0 to 11.5. One solution would be to recover the finely divided co-product crystals from the aqueous effluent and then reconstitute them into microspheres by addition of a binder and subsequently ion-exchange the resulting microspheres to provide fluid cracking catalysts. It is well known, however, that it is difficult to produce microspheres that are both high in zeolite content and also sufficiently resistant to attrition to be used in modern FCC units. See U.S. Pat. No. 4,493,902. Consistent with this, we have found that it is more difficult to produce bonded catalysts which are highly resistant to attrition when using finely divided co-products of U.S. Pat. No. 4,493,902 as compared to the finely divided co-products of Ser. No. 532,346.
The microspheres of calcined clay used as reactants in the process of U.S. Pat. No. 4,493,902 have an average diameter of about 60 to 70 microns and contain a minimal amount, e.g., 3-5% weight percent, of particles finer than 20 microns, equivalent spherical diameter. Typically, the largest particles have a diameter of about 150 microns. The reason for restricting fines in the microspheres utilized as a reactant is that the microspheres will retain substantially the same size and shape of the original microspheres during aging and crystallization. The content of particles finer than about 20 microns in the finished crystallized catalyst product should be limited because it is difficult or impossible to retain such particles in fluid catalytic cracking units. Furthermore, fines introduced with the reactants and/or generated during processing interfere with the operation of filtration equipment used in carrying out the ion-exchange treatment employed to convert the crystallized microspheres into catalytically active and selective particles.
When some of the production schemes described in U.S. Pat. No. 4,493,902 are conducted on a commercial scale, fines (e.g., particles finer than about 20 microns equivalent spherical diameter) may be generated during aging and/or crystallization. The amount of such fines may vary, depending, among other things, on the type and quality of the seeds as well as the attrition-resistance of the precursor microspheres and their response to aging and crystallization. These fines typically contain about 50 to 70 percent sodium zeolite Y (as estimated by quantitative X-ray diffraction determinations). There is an indication that at least part of the zeolite Y in the fines results from a chemical reaction carried out in the aqueous phase (in contrast to zeolite that is present as a result of breakdown of crystallized microspheres). In some cases, small amounts of the zeolite having the X-ray pattern of sodium zeolite B may also be contained in the fines. Also present is amorphous silica-alumina derived at least in part from calcined clay. A small amount of filter aid material (e.g., diatomaceous earth) is normally present in fines from a commercial plant. The origin of such material will be explained subsequently.
The fines generated as a co-product stream are advantageously removed from the mainstream of crystallized microspheres before the crystallized microspheres undergo filtration to remove the mother liquor. Removal of the fines can be accomplished by passing the slurry of crystallized microspheres and mother liquor through one or more hydroclones before the slurry undergoes filtration. In the hydroclones, grade material is discharged as the underflow effluent and is fed to the deliquoring filter. The overflow effluent from the primary hydroclones is combined with the filtrate from the deliquoring filter and run through a secondary hydroclone. The underflow again goes to the deliquoring filter and the overflow becomes the "unclarified" sodium silicate solution. Mother liquor is separated from the fines for eventual reuse, concentration and sale. Such a concentrated mother liquor by-product typically contains about 15% by weight Na.sub.2 O, 29% by weight SiO.sub.2 and 0.1% Al.sub.2 O.sub.3, the balance being water. In other words, the concentrated mother liquor has a composition approximating that of sodium disilicate and has a concentration of about 44% by weight.
A conventional rotating drum filter precoated with a filter aid such as diatomaceous earth is used to remove the fines from the alkaline sodium silicate liquor. The fines build up on the filter and are gradually scraped as a moist cake from the surface of the drum. The cake removed from the filter is also associated with entrained sodium disilicate solution (or solution having a Na.sub.2 O/SiO.sub.2 ratio similar to that of the disilicate), typically in amount corresponding to about 3-5% SiO.sub.2 (weight basis), based on the dry weight of the fines. The material removed from the filter has been handled in the past as waste material, creating a potential disposal problem. It is this material that is converted to an active cracking catalyst by practice of the present invention.
Procedures for preparing fluid zeolitic cracking catalyst particles that involve mixing sodium silicate solution with zeolite crystals and spray drying the slurry to form microspheres are known. Reaction products containing synthetic faujasite and obtained from sodium hydroxide solution and a mixture of calcined kaolin clay are used in processes described in the following: U.S. Pat. Nos. 3,515,683; 3,451,948; and 3,458,454 all assigned to Air Products and Chemicals, Inc. In these processes the zeolite-containing reaction products are ground before spray drying. A grinding step is also utilized when the feed to the spray dryer is obtained by reacting calcined clay and sodium hydroxide solution in the absence of hydrated clay. See U.S. Pat. No. 3,515,682, also assigned to Air Products and Chemicals, Inc. In U.S. Pat. No. 3,451,948 ion-exchange after spray drying is carried out by neutralizing the slurry at pH 5.5-8.5 at 30.degree.-40.degree. C., in the absence of ammonium nitrate, followed by ion-exchange at pH 5.5-8.5 at 30.degree.-40.degree. C. The patent teaches that pH should be maintained above 4.
Attempts by the coinventors in the subject application to bind the drum fines produced as a side stream by the embodiment of U.S. Pat. No. 4,493,902 using internal zeolite growth seeds (e.g., Example 3 of the '902 patent) were moderately successful in producing a product having a satisfactorily low attrition value. Using a sodium silicate binder and spray drying, the attrition value was almost 2%/sec when measured by the EAI (Engelhard Attrition Index) test cited in U.S. Pat. No. 4,493,902. This value is similar to that of some present day commercial FCC catalyst but is above the EAI maximum value of 1%/sec which we preferred to produce. Some improvements resulted when hydrous kaolin was introduced into the spray dryer feed but desired EAI values of 1%/sec or below were not achieved. Surprisingly, by using fines obtained from externally seeded processed feed instead of internally initiated feed, attrition-resistant catalyst particles were produced in numerous runs; a single run resulting in an EAI as high as 1.9%/sec was an exception.
We have also found that the procedures used in the ammonium ion-exchange treatment to reduce sodium in the spray dried particles should be carefully controlled, as described hereinafter, in order to obtain products of desired hardness.